In automotive lighting applications, several different types of lamp can be used to generate a beam in front lighting applications. For example, filament lamps such as the H7 lamp are widely used in reflector assemblies that use a reflector to shape the beam, while discharge lamps such as the D4 lamp are well known for use in projector headlamps that use a lens as well as a reflector to shape the beam. Filament lamps such as the H-series lamps generate a mainly white light that may be perceived to be yellowish in colour. In contrast, the more modern high-intensity discharge (HID) lamps (often simply referred to as “xenon” lamps) generate a very bright light with a distinct blue hue. However, an existing filament lamp cannot simply be replaced by a better and more xenon lamp, since the reflector for a filament lamp is only approved for those filament lamp types covered by the UNECE (United Nations Economic Commission for Europe) regulations such as regulation R37. Therefore, some efforts have been invested in altering the exterior of a conventional lamp to modify its beam properties. For example, a prior art lamp has been modified to include a coating in a region of the lamp close to the base, leaving the rest of the lamp surface uncoated. The purpose of the coating is to alter the colour of the light emitted towards the base of the lamp while leaving the beam unaffected. Other lamps are known in an interference filter is applied to the surface of the lamp to alter the beam. However, such an interference filter is quite expensive to apply, since several very thin but very uniform layers in the region of a few tens of nanometers must be applied in succession.
Therefore, it is an object of the invention to provide a more efficient and robust way of modifying the light and colour output of a lamp while complying with the relevant regulation.